This invention relates to a current transformer, and more particularly it relates to a self-cooled, high-voltage current transformer which is cooled by the circulation of a dielectric fluid through a tubular primary conductor.
It is well-known in the art to cool the primary winding of a high-voltage current transformer by the provision of cooling passages within the primary winding through which a dielectric fluid is made to circulate by convection. The general structure of this type of self-cooled current transformer is illustrated in FIGS. 1 through 3 of the attached drawings, which show a current transformer disclosed in Japanese Published Patent No. 43-3129 (1968). FIG. 1 is a vertical cross-sectional view thereof, while FIGS. 2 and 3 are enlarged transverse cross-sectional views taken along Line II--II and Line III--III, respectively, of FIG. 1. As shown in the figures, a primary conductor 1 of the current transformer comprises a straight portion and an annular portion. The straight portion comprises an inner tubular conductor 2 and an outer tubular conductor 3 which is concentrically disposed with respect to the inner conductor 2. The inner conductor 2 is longer than the outer conductor 3 and extends past both ends of the outer conductor 3. The outer surface of the inner conductor 2 is conveyed by an electrically-insulating tube 4 which electrically insulates the inner conductor 2 and the outer conductor 3 from one another. The annular portion of the primary conductor 1, which serves as a primary winding, comprises an annular conductor 6 which has two parallel channels 5 formed in one surface. The annular conductor 6 is bent into roughly the shape of the letter C with the channels 5 on the outside thereof. One end of the annular conductor 6 is welded or brazed to the bottom end of the outer conductor 3 while the other end is similarly connected to the bottom end of the inner conductor 2 so that the channels 5 of the annular conductor 6 communicate with the insides of the inner and outer conductors 2 and 3. The entire outer surface of the primary conductor 1 is electrically insulated by main electrical insulation 7. The annular conductor 6 is interlinked with a laminated core 8 around which a secondary winding 9 is wrapped. The secondary winding 9 is electromagnetically coupled with the annular portion 6 of the primary conductor 1 so that a current flowing through the primary conductor 1 will induce a secondary current in the secondary winding 9.
The open upper ends of the inner and outer conductors of the primary conductor 1 are disposed inside an unillustrated tank filled with a dielectric fluid so that the dielectric fluid can flow through the entire length of the primary conductor 1. The outer conductor 3 is designed to have a lower current density than the inner conductor 2, with the result that when a current flows through the primary conductor 1 during operation of the transformer, the temperature of the inner conductor 2 is higher than that of the outer conductor 3. This temperature difference produces convection of the dielectric fluid within the primary conductor 1. Namely, as shown by the arrows in FIG. 1, the dielectric fluid flows downwards from the unillustrated tank and enters the space between the inner conductor 2 and the outer conductor 3, flows down the length of the outer conductor 3 and enters the channel 5 formed in the annular conductor 6, flows through the length of the annular conductor 6 and enters the hollow center of the inner conductor 2, rises the length of the inner conductor 2, and flows back into the tank, thereby cooling the primary conductor 1.
The structure of the conventional current transformer illustrated in FIG. 1 has the drawback that the primary conductor 1 comprises three separate members (the inner conductor 2, the outer conductor 3, and the annular conductor 6) which must be assembled and then joined to one another by the labor-intensive processes of welding or brazing, which increase manufacturing costs. Furthermore, at the connection between the bottom ends of the inner conductor 2 and the outer conductor 3 and the annular conductor 6, the flow path of the dielectric fluid bends 90 degrees since the ends of the annular conductor 6 are at right angles to the bottom ends of the inner and outer conductors. These 90-degree bends increase the resistance to flow, with the result that the circulation of the dielectric fluid is decreased and the cooling effect provided thereby may be inadequate. Furthermore, as the dielectric fluid can not flow smoothly past these sharp bends, there are regions within the fluid in the vicinity of the bends in which the fluid is largely stagnant, providing almost no cooling effect and resulting in the localized heating of the conductors.